Apparatus for crane jibs adapted to be extended and retracted telescopically

ABSTRACT

A crane jib for attachment to a crane column including in combination a jib inner section pivotally attached to the crane column, a jib central section telescopically disposed with respect to the inner section, a jib outer section telescopically disposed with respect to the central section, first and second hydraulic cylinders disposed within the central section and having their rods attached respectively to the inner and outer sections, valve means and control means for applying fluid pressure through the valve means to the hydraulic cylinders for telescoping said sections substantially simultaneously for extension and retraction of the jib.

United States Patent Sundin [151 3,672,159 1 51 June 27, 1972 [s41APPARATUS FOR (IRANE JIBS 2,984,373 5/1961 Przybylski ..214/141 ADAPTEDTO BE EXTENDED AND 3,212,604 /1965 Gamett ..9l/l73 X RETRACTEDTELESCQPICALLY 3,240,008 3/1966 McMullen ..60/ 10.5

3,250,182 5/1966 Mansel ..9l/l67 [72-] inventor: Anders Erlk Sundln,Forsa, Sweden 3 300 050 /19 7 Lado I 212 3 5 [73] Assignees 1 A. E.Sundln Produkter AB, Delsbo, 3'407947 10/1968 vaua "91/41 1 X SwedenPrimary Examiner-Martin P. Schwadron [22] Flled: 1 1970 AssistantExaminer-Irwin C. Cohen [21] APPL No: 13,991 Attorney-Kane, Dalsimer,Kane, Sullivan and Kurucz [57] ABSTRACT Foreign Application PriorityData A crane jib for attachment to a crane column including in Feb. 28,I969 Sweden ..2764/69 combination a jib inner section pivotally attachedto the crane column, a jib central section telescopically disposed with[52] US. Cl 60/105, 91/4] 1, 9211/3/35? respect to the inner section a jouter section telescopically 58 Field ofSeai-ch....; ..60/10.5-187/9-91/167 168 y I 91/4 I 436 2 2 havmg the1r rods attachedrespectlvely to the 1nner and outer sections, valve means and controlmeans for applying fluid [56] keferencescned tpressure through the valvemeans to the hvdraulic cylinders or telescoplng said sect1onssubstantially s1multaneously for UNITED STATES PATENTS extension andretraction of the jib.

2,646,025 7/1953 Deardorff ..9l/436 3Claims,5DrawingFlgur-es I\ I\I\7\IR \ll 1\ Ik PATENTEnJum 1972 3.672.159

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INVEN TOR ANpERs ER\K suuom augzilfi z wwa ATTORNE Y5 APPARATUS FORCRANE JIBS ADAP'IED TO BE EXTENDED AND RETRACTED TELESCOPICALLYBACKGROUND OF THE INVENTION It is very difficult in the case of cranejibs adapted to be extended and retracted telescopically, moreparticularly in the case of crane lorries, to obtain a satisfactoryassociation of the displacement movements of the individual telescopicsections, more particularly if the crane jib comprises at least threetelescopic sections. It is not possible to achieve a mutualco-ordination between the extended telescopic sections of telescopicallyextensible and retractable crane jibs in advance, whose telescopic unitscomprise only one common pressure container to which a hydraulic fluidis supplied. Also, the telescopic sections are extended individually,which often results in unnecessary high bearing pressure in theindividual sections and at the same time causes a non-uniform extendingmovement.

In order to obtain uniform load distribution over the bearings of theindividual telescopic sections, it is desirable to have a substantiallysimultaneously occurring extension or retraction of the telescopicsections, and in some cases this is even necessary, and yet their speedof extension and length of travel may be different.

It has already been attempted to solve this problem by a number ofdifferent constructions, but all of these more or less involvedisadvantages and defects. For example there has been provided a cranejib which is telescopically extensible and comprises a double-actinghydraulic cylinder, but the latter only operates directly one of thetelescopic sections at a time. The retracting and extending movement ofthe other telescopic sections is brought about by means of a verycomplicated rope system with running wheels arranged at the outside ofthe crane jib. However, the tractive force when extending or shorteningthe jib by means only a short working life, since it is easily damaged.

It is also already known to provide each individual telescopic sectionwith a hydraulic unit, these hydraulic units being connected in parallelwith one another and connected to an actuating device. But parallelarrangement of the hydraulic units involves the disadvantage that boththe magnitude of the force and the rate of movement of the hydrauliccylinders are different in the two directions of movement. Accordingly,the telescopic sections are not extended simultaneously, and instead thesection which has the least frictional resistance is extended first.Since the sequence in which the telescopic sections are extended is notcorrectly predetermined, the guiding of the hose for the hydraulic fluidwithin the crane jib is made substantially more difficult. If forexample the foremost jib section is extended first, the hydraulic fluidhoses must be capable of being extended in order not to be fractured atthe rear edge of the central section.

SUMMARY OF THE INVENTION Both for the solution of these problems and inorder to obviate the aforesaid disadvantages, an apparatus has beenprovided which is characterized in that the two hydraulic cylinders aresecured to an end wall and situated within the central telescopicsection, the piston rod of the one (first) hydraulic cylinder beingsecured to the end of the inner telescopic section pivotably arranged onthe crane column and the piston rod of the second hydraulic cylinderbeing secured to the end wall of the outer telescopic section, and inthat the hydraulic cylinders are connected in series in a pressure fluidsystem.

The apparatus according to the invention is also characterized byelements which, for extension of the crane jib, conduct a pressure fluidfrom a pressure source by way of a first pressure fluid supply conduitdirectly into the inner pressure chamber i.e. the pressure chambersituated at that end of the second hydraulic cylinder which is nearestto the crane column, the piston and the piston rod of this hydrauliccylinder being displaced outwardly and pressure fluid present in theouter pressure chamber of this second pressure cylinder being pressedout through an outer communicating conduit and supplied to the outerpressure chamber of the first hydraulic cylinder, whereby this cylinderis displaced relatively to its piston and by means of the said pistonpressure fluid present in the inner chamber of the first hydrauliccylinder is pressed by way of an inner communicating conduit, through avalve arrangement and a pressure fluid duct and then introduced into theinner pressure chamber of the second hydraulic cylinder.

The apparatus according to the invention is also characterized byelements which in order to shorten the crane jib, introduce a pressurefluid from a pressure source by way of a second pressure fluid supplyconduit, through the valve arrangement and the forward communicatingconduit and into the inner pressure chamber of the first hydrauliccylinder that is displaced inwards relatively to its piston and pressingpressure fluid through the outer communicating conduit into the outerpressure chamber of the second hydraulic cylinder, whereby the piston ofthis hydraulic cylinder is displaced inwards relatively to its cylinder,and pressure fluid present in the inner pressure chamber is pressedthrough the first pressure fluid supply conduit, serving as an inletwhen the jib is extended, and supplying it to a pressure fluid containerof the pressure source.

Further features of the invention will become apparent from thefollowing.

An apparatus according to the present invention makes it possible toeffect a substantially simultaneous extension and also a substantiallysimultaneous retraction of the individual telescopic sections, andfurthermore e.g. the outer most telescopic section can be given, byspecial choice of dimensions, a greater travel length and a higher speedof displacement than the telescopic section adjacent it. As a result,the pressure produced by the lifted load and acting on the bearing partsof the hydraulic section is uniformly distributed, so that at theextension or retraction of the jib the bearing pressure is kept as lowas possible in every section. Owing to the fact that the outermosttelescopic section is given a greater travel length, the telescopicsections situated nearer the crane column are given larger bearingsurfaces, so that a still more uniform distribution of the bearingpressure in the sections is achieved.

Owing to the direct action of force by the hydraulic cylinders on thetelescopic sections and the symmetry thereof in mechanical movement, aconsiderable tractive force is obtained in the longitudinal direction ofthe crane jib both at extension and at retraction of the jib. Also themovement during extension or retraction of the jib is-very uniform inall telescopic sections.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view, partly insection, of a crane jib according to the invention with the telescopicsections fully extended,

FIG. 2 shows a longitudinal sectional view through the crane jib in theretracted state,

FIG. 3 shows the hydraulic system during the extension of the jib, in adiagrammatic view,

FIG. 4 is again a diagrammatic view, showing the hydraulic system duringthe retraction of the crane jib and FIG. 5 shows a partial elevationalview of a further form of embodiment, illustrating the bearing ormounting of the pressure cylinders on one of the telescopic sections.

DETAILED DESCRIPTlON OF A PREFERRED EMBODIMENT According to FIG. 1, inone example of embodiment of the apparatus according to the inventionthe crane jib l is mounted to be pivotable on a crane column 2 which isarranged to be rotatable for example on the back of a lorry for pivotingthe crane jib 1 in the horizontal plane. The crane jib l is adapted tobe lifted and lowered by means of a hydraulic cylinder 3 and consists ofthree telescopic sections 4,5,6 which are conveniently of rectangularcrosssection and of which the inner telescopic section 4, mountedpivotably on the crane column 2, is not mobile in the longitudinaldirection of the crane jib l. The two telescopic sections and 6 arearranged to be displaceable whithin one another and to be capable ofbeing contracted into extended from the telescopic section 4. The outertelescopic section 6 is provided at its end wall 21 with a load hook 17on which the load to be lifted is suspended either directly or by meansof a rope, to be then loaded for example on to a motor lorry. The innerand central telescopic sections 4 and 5 are provided within their outerend respectively both above and below with shoulders 43, 43', 44, 44'which serve together with shoulders 45, 45', 46, 46' (see FIG.2)arranged on the inner end of the central and outer telescopic sections5, 6 above and below and extending radially outwardly, as supportingsurfaces, guide shoulders and end stops. However, these may also bemounted supporting rollers. The extending and retracting movements ofthe crane jib are carried out by means of a hydraulic system comprisingchiefly two hydraulic cylinders 7,8, pressure fluid being supplied tothe said hydraulic cylinders at the extension and retraction of thecrane jib l by means of a hydraulic pump 9 from a pressure fluidcontainer 10 by way of a changeover device 11 and a valve arrangement 12(see FIG. 2). FIG. 2 of the drawings also shows that the two hydrauliccylinders 7, 8 are arranged with their end nearest to the crane column 2at the end wall 13 of the central telescopic section 5. The hydrauliccylinder 7 (in future referred to as the first hydraulic cylinder) isprovided with a piston 14 and a piston rod 15 which extends through theend wall 13 of the central telescopic section 5 and at its inner end,nearest the crane column 2, is secured to the end 16 of the innertelescopic section 4 ar ranged pivotably on the crane column 2. Thesecuring element 18'for the hydraulic cylinder 7,- serving as a support,must, owing to the unavoidable deflection of the telescopic sections 5,6when the crane jib is subjected to considerable load, be made to besomewhat yieldable in a plane transversely to the direction ofdisplacement of the crane jib 1. The second hydraulic cylinder 8 isprovided with a piston 19 and a piston rod 20 whose outer end is securedto the end wall 21 of the outer telescopic section 6. The inner end,nearest to the crane column 2, of the hydraulic cylinder 8 (designatedhereinafter as the second hydraulic cylinder") is also mounted in theend wall 13 of the central telescopic section 5, as shown in FIG. 2.

As for example FIGS.3 and 4 show diagrammatically, the hydraulic systemcomprises the aforesaid pressure fluid container 10, from which thepressure fluid is conveyed by means of the hydraulic pump 9 to thechange-over device 11, which changeover device is adjustable to threedifferent positions, namely one position for extension of the crane jib1, one position for retraction of the said jib, and an intermediateposition wherein the pressure fluid is returned directly to the pressurefluid container 10 without influencing the two hydraulic cylinders 7,8.From the change-over device 11, two pressure fluid supply conduits 22,23 lead to the crane jib 1, the first of which 22 is connected directlyto the second hydraulic cylinder 8 and the second 23 to a double-actingnon-retum valve 24, which valve is arranged in an inner end portion 25of the second hydraulic cylinder 8 which is nearest the crane jib 2.This double-acting non-return valve is formed by a valve ball 27 whichis mobile in a valve chamber 26 and which blocks the second pressurefluid supply conduit 23 and a pressure fluid duct 28 situated betweenthe valve chamber 26 and the first pressure fluid supply conduit 22.Leading to the valve chamber 26 is a further pressure fluid conduitwhich is referred to hereinafter as the inner communicating conduit 29,which is constantly open and connects the valve chamber 26 to the innerpressure chamber 30, nearest to the crane column 2, of the firsthydraulic cylinder 7. The two outer pressure chambers 31, 32, nearest tothe load hook 17 (see FIG. 2), of the hydraulic cylinders 7,8 areconnected by a pressure fluid conduit, in future referred to as theouter communicating conduit 33, to one another. The piston 14 of thefirst hydraulic cylinder 7 is also provided with a throughflow duct 34in the outer end of which there is arranged a spring-loaded valve 35with a projection 36 which, when the piston 14 has reached its outer endposition, abuts against the end wall 37 of the hydraulic cylinder 7 andholds the valve 35 open. At the inward displacement of the hydrauliccylinder 7, the valve 35 is closed by the action of the pressure spring38 associated therewith.

FIG. 5 shows a partial view, shown in longitudinal section, of themounting or securing of the hydraulic cylinders according to a furtherform of embodiment, which has a few constructional advantages relativelyto the form of embodiment described hereinbefore. In this form ofembodiment the inner end portion 18, 25, nearest to the crane column 2,of the two hydraulic cylinders 7, 8 is constructed as a single endportion 39 and provided with a common mounting portion 40 which isarranged in the end wall 13 of the central telescopic section 5 and ismobile in a plane transversely to the longitudinal direction of thecrane jib 1. The pressure fluid duct 28 opens directly into the innerpressure chamber 41 of the hydraulic cylinder 8 and the non-return valve24 with the connections is so arranged that the valve ball 27 carriesout a horizontal movement.

By way of a summary, the course of the extension operation (se FIG. 3)and the retraction operation (see FIG. 4) of the crane jib 1 will bedescribed hereinafler,

When using the crane and when the crane jib 1 is not extended orretracted, the change-over device 11 is at neutral, the hydraulic pump 9delivering the pressure fluid directly back to the pressure fluidcontainer 10. When it is necessary to extend the crane jib I, theoperating lever 42 of the changeover device 1 l is swung to the positionshown in FIG. 3. In this position, the pressure fluid is conductedthrough the first pressure fluid supply conduit 22 and into the innerpressure chamber 41 of the second hydraulic cylinder 8, whereby thepiston 19 is pressed outwards. The piston rod 20 arranged in astationary position on this piston and the outer telescopic section 6are now shifted outwards. At the same time pressure fluid is alsoconducted through the outer communicating conduit 33 and into the outerpressure chamber 31 of the first hydraulic cylinder 7. Since the piston14 of the first hydraulic cylinder 7 is situated in the end position,the valve is open and the pressure fluid can consequently pass throughthe throughflow duct 34. As a result, a delay is achieved in theextension of the central telescopic section 5, but this depends on thesize of the cross-sectional area of the throughflow duct 34. With arelatively small cross-sectional area the two cylinders 7,8 are set inmotion substantially simultaneously, the first cylinder 7 also beingdisplaced relatively to the piston 14 associated therewith. Since thepiston rod 15 with its inner end, directed towards the crane column 2,is secured to the end 16 of the inner telescopic section 4, which is notmobile in the direction of prolongation of the jib, the centraltelescopic section 5 is moved outwards. In this case pressure fluid ispressed into the inner pressure chamber 30 of the first hydrauliccylinder 7, through the inner communicating conduit 29 and introducedinto the valve chamber 26 of the non-return valve 24, the valve ball 27blocking the second pressure fluid supply conduit 23, which is thuscompletely inoperative. The pressure fluid is therefore conductedthrough the pressure fluid duct 28 and into the inner pressure chamber41 of the second hydraulic cylinder 8 again.

At retraction of the crane jib l, the operating lever 42 of thechange-over device I 1 is swung to the position shown in FIG. 4. Whenthis happens, pressure fluid is introduced through the second pressurefluid supply conduit 23 and into the valve chamber 26 of the non-returnvalve 24, the valve ball 27 blocking the pressure fluid duct 28 and thepressure fluid consequently being introduced through the innercommunicating conduit 29 and into the inner pressure chamber 30 of thefirst hydraulic cylinder 7. Since the piston rod 15 is securely arrangedat one of its ends on the end of the telescopic section 4 pivotablymounted on the crane column 2, the cylinder 7 is displaced inwardsrelatively to its piston i.e. the central telescopic section 5 isretracted. Pressure fluid is introduced through the outer communicatingconduit 33 into the outer pressure chamber 32 of the second hydrauliccylinder 8, whereby the piston 19 with the piston rod 20 associatedtherewith and with it also the outer telescopic section 6 are shiftedinwards i.e. retracted. The pressure fluid present in the inner pressurechamber 41 of this hydraulic cylinder 8 is conveyed through the firstpressure fluid supply conduit 22, which is connected by the change-overdevice 11 to the pressure fluid container 10, back into the pressurefluid container which itself communicates with the hydraulic pump 9. Assoon as the piston 14 of the first hydraulic cylinder 7 has reached itsouter end position, the valve 35 is opened, the pressure fluid beingpressed through the throughflow duct 34 and flowing into the secondhydraulic cylinder 8 until the piston 19 thereof has reached its innerend position. The travel length of the hydraulic cylinders 7,8 and theireffective cross-sectional areas can be so chosen that the piston 14 ofthe first hydraulic cylinder 7 reaches its end position before thepiston 19 of the second hydraulic cylinder 8 has reached its endposition i.e. the central telescopic section 5 has already run in beforethe running-in movement of the outer telescopic section 6 terminates. Ifthe apparatus is so dimensioned that the pistons 14, 19 reachtheirparticular end positions simultaneously, relatively small displacementsmay occur in time owing to leakage at the piston seals. But the valve 35guarantees that the two pistons 14, 19 actually occupy their relevantend positions. Since the two outer pressure chambers 31, 32 of thehydraulic cylinders 7, 8 form with the outer communicating conduit 39 aclosed hydraulic system, the valve 35 is extremely important for thefilling of the system with pressure fluid.

in order to enable the apparatus according to the invention to besuitably dimensioned for obtaining a predetermined travel length for thetwo hydraulic cylinders 7,8, a mathematical relationship is necessarybetween on the one hand the cross-sectional areas of the cylinders andthe piston rods and on the other hand their travel length, which isexplained hereinafter in more detail:

a the inner cross-sectional area of the first hydraulic cylinder 7 bcross-sectional area of the piston rod c the inner cross-sectional areaof the second hydraulic cylinder 8 d cross-sectional area of the pistonrod 20 e travel length of the first hydraulic cylinder 7 f= travellength of the second hydraulic cylinder 8.

If no pressure fluid leaves the pressure fluid system during theextension of the jib, the introduced quantity of pressure fluid isidentical with the decrease in the volume of the parts of the pistonrods situated in the cylinders, from which the following equationfollows (d f b e).

In order that during retraction, with the same pressure, a tractiveforce can be obtained in the piston rods 15, 20 which is equal inmagnitude to the compressive force thereof during extension, the samequantity of fluid must be supplied. During retraction, in addition tothe stored quantity of fluid the quantity of fluid displaced by themoved-in piston rods 15, 20 must also leave the hydraulic cylinders 7,8.Therefore the following equation is obtained:

A certain deviation from the dimensional relationships is permitted inpractice in order to adapt to existing standard dimensions and auxiliarymeans used in production.

If both hydraulic cylinders have the same travel length (e f) and thepiston rod cross-sectional areas are the same in magnitude (b d), thereis obtained The decrease in the quantity of fluid in the outer pressurechamber 32 of the second hydraulic cylinder 8 is the same as theincrease in the fluid quantity in the outer pressure chamber 31 of thefirst hydraulic cylinder 7, i.e.

(c d) f a e But if there is the same travel length (e =f) and piston rodcross-sectional area (b =d), then according to the foregoing c 4d. Theintroduction of these values into the previous equation gives Since thedifl'erence between the travel length of the two cylinders is generallynot greater than about 10 20 percent of the travel length in order tosubject the bearings of the crane jib 1 to loads of equal magnitude, thelast-indicated equations can be used in a preliminary calculation forcomputing the measurements of the apparatus.

The apparatus according to the invention provides a crane jib which isvery compact in the retracted position in relation to its operatingcircle, which is very advantageous when used for example in the case ofcrane lorries, since cranes used in these are usually arranged, duringtransport, behind the drivers cab transversely to the longitudinaldirection of the lorry, and for this reason should be somewhat less thanthe overall width of the vehicle.

The forms of embodiment illustrated and described should only beregarded as examples and can be varied construc tionally in various wayswithin the framework of the following claims. For example the piston ofthe second hydraulic cylinder can be provided with a throughflow ductand a valve. Finally, several apparatus according to the presentinvention can be connected following one another in a telescopicallyreu'actable and extensible crane jib, in which case, for shifting forexample six telescopic units, a hydraulic cylinder pair according to thepresent invention is arranged in the second telescopic section and afurther pair of hydraulic cylinders within the fifth telescopic section.

I claim:

1. A crane jib for attachment to a crane column including in combinationa jib inner section pivotally attached to said crane column, a jibcentral section telescopically disposed with respect to said innersection, a jib outer section telescopically disposed with respect tosaid central section, first, second and third interior walls of said jibinner, central and outer sections respectively, first and secondhydraulic cylinders rigidly attached to said second wall, first andsecond piston rods respectively of said first and second hydrauliccylinders attached respectively to said first and third walls, a firstpiston head mounted on said first piston rod within said first cylinder,inner and outer first cylinder chambers separated by said first pistonhead, a second piston head mounted on said second piston rod within saidsecond cylinder, inner and outer second cylinder chambers separated bysaid second piston head, an outer communicating conduit connecting saidouter cylinder chambers, a flow-through duct in said first piston headconnecting said inner and outer first cylinder chambers, a valve withinsaid flow-through duct, a double-acting non-retum valve, a valve chamberof said double-acting valve, an inner communicating conduit connectingsaid inner chambers through said valve chamber, a fluid pressure source,a first pressure fluid conduit connecting said inner second cylinderchamber through said valve chamber with said pressure source, a secondpressure fluid conduit connecting said inner first cylinder chamber withsaid pressure source through said valve chamber, control meansselectively determining the entry of pressure into said pressure fluidconduits from said pressure source whereby upon extension of said jibpressure is conducted by said first pressure fluid conduit into saidinner second cylinder chamber displacing said second piston head andpressing pressure from said outer second cylinder chamber through saidouter communicating conduit into said outer first cylinder chamberdisplacing said first piston head and fluid present in said inner firstcylinder chamber is pressed into said inner second cylinder chamberthrough said inner communicating conduit and said valve chamber andwhereby upon retraction of said jib pressure is conducted by said secondpressure fluid supply into said inner first cylinder chamber throughsaid valve chamber and said inner communicating conduit displacing saidfirst piston head and pressing pressure through said outer communicatingconduit into said hydraulic cylinders are rigidly attached and saidmember is pivotally attached to said second wall mobile in a planetransversely to the longitudinal direction of said crane jib.

3. A crane jib in accordance with claim 1 in which the valve within saidflow-through spring loaded member so that at the outer end position ofsaid piston head said inner and outer first cylinder chambers areconnected through said flow-through duct.

1. A crane jib for attachment to a crane column including in combinationa jib inner section pivotally attached to said crane column, a jibcentral section telescopically disposed with respect to said innersection, a jib outer section telescopically disposed with respect tosaid central section, first, second and third interior walls of said jibinner, central and outer sections respectively, first and secondhydraulic cylinders rigidly attached to said second wall, first andsecond piston rods respectively of said first and second hydrauliccylinders attached respectively to said first and third walls, a firstpiston head mounted on said first piston rod within said first cylinder,inner and outer first cylinder chambers separated by said first pistonhead, a second piston head mounted on said second piston rod within saidsecond cylinder, inner and outer second cylinder chambers separated bysaid second piston head, an outer communicating conduit connecting saidouter cylinder chambers, a flow-through duct in said first piston headconnecting said inner and outer first cylinder chambers, a valve withinsaid flow-through duct, a double-acting non-return valve, a valvechamber of said double-acting valve, an inner communicating conduitconnecting said inner chambers through said valve chamber, a fluidpressure source, a first pressure fluid conduit connecting said innersecond cylinder chamber through said valve chamber with said pressuresource, a second pressure fluid conduit connecting said inner firstcylinder chamber with said pressure source through said valve chamber,control means selectively determining the entry of pressure into saidpressure fluid conduits from said pressure source whereby upon extensionof said jib pressure is conducted by said first pressure fluid conduitinto said inner second cylinder chamber displacing said second pistonhead and pressing pressure from said outer second cylinder chamberthrough said outer communicating conduit into said outer first cylinderchamber displacing said first piston head and fluid present in saidinner first cylinder chamber is pressed into said inner second cylinderchamber through said inner communicating conduit and said valve chamberand whereby upon retraction of said jib pressure is conducted by saidsecond pressure fluid supply into said inner first cylinder chamberthrough said valve chamber and said inner communicating conduitdisplacing said first piston head and pressing pressure through saidouter communicating conduit into said outer second cylinder chamberdisplacing said second piston head, and said non-return valve includinga free-moving valve means in said valve chamber, said free moving valvemeans alternatively blocking said first and second pressure fluid supplyconduits communicating with said valve chamber and constantlymain-taining the inner communicating conduit open.
 2. A crane jib inaccordance with claim 1 in which a member is provided in which saidvalve chamber and said inner communicating conduit are formed and towhich said hydraulic cylinders are rigidly attached and said member ispivotally attached to said second wall mobile in a plane transversely tothe longitudinal direction of said crane jib.
 3. A crane jib inaccordance with claim 1 in which the valve within said flow-throughspring loaded member so that at the outer end position of said pistonhead said inner and outer first cylinder chambers are connected throughsaid flow-through duct.